1. Field of the Invention
The present invention relates to a solid state image sensor and, more particularly, to a black-level signal generation circuit employed in a CMOS (Complementary Metal-Oxide-Semiconductor) image sensor as a solid state image sensor.
2. Description of the Related Art
A CMOS image sensor can be fabricated by use of the CMOS process for logic LSIs. Therefore, the CMOS image sensor contains, on a single semiconductor chip, photo-sensor units or pixels and a signal processing circuit such as control logic, unlike a CCD-based image sensor. Furthermore, the CMOS image sensor has an advantage that it has lower noise than usual MOS-based image sensors, because each cell or pixel includes a photodetector and a charge-to-signal conversion circuit so that the conversion of the charge detected by the photodetector in response to light irradiated thereto into an electrical signal as well as the amplification of the signal thus converted are performed within each pixel.
This kind of image sensor is described, for example, by Sunetra K. Mendis et al., xe2x80x9cA 128xc3x97128 CMOS Active Pixel Image Sensor for Highly Integrated Imaging Systemxe2x80x9d (IEEE International Electron Devices Meeting 93, pp. 583-586) and by Roger A. Panicacci et al., xe2x80x9c128 Mb/s Multiport CMOS Binary Active-Pixel Image Sensorxe2x80x9d (1996 IEEE International Solid-State Circuits Conference pp. 100-101). Methods of reading out pixel information of an image sensor as described in these references are as follows.
In the Mendis et al. CMOS sensor, charge responsive to a light that has been illuminated to a photodetector for a given time is read out as an electrical signal onto a vertical signal line through a first source-follower consisting of an input transistor and a row-selection transistor. The potential of the vertical signal line is sampled on a capacitor by enabling a sample switch. Each potential at each capacitor is read out through second and third source-followers respectively, by enabling column-selection transistors attached to these source-followers. An image signal is created using these two kinds of voltage signals.
In the Panicacci et al. CMOS sensor, on the other hand, pixels are formed in the same way as the Mendis et al. pixels. However, the circuit of a readout portions has a signal detection capacitor whose one end is connected with a sample switch, the other end being connected with the gate of the input transistor of an output side source-follower. The photodiode is exposed to a light for a given time. Then, the voltage across this photodiode is read out into a column signal line through an input side source-follower. This voltage is sampled in the signal detecting capacitor by enabling the sample switch and is read out through the output side source-follower. Then, this voltage is compared with an externally applied threshold voltage. Required pixel information is obtained using this thresholded voltage.
Generally, a signal processing circuit of a image sensor needs a black-level signal forming a reference potential. Usual method of forming and way of using such a black-level signal are next described by referring to FIG. 1, which is a block diagram of the conventional MOS-based image sensor, in which a source-follower input transistor is included within each pixel. In FIG. 1, pixels are arranged in a plurality of rows and a plurality of columns to form an effective pixel array 111. A vertical scanning circuit 113 selects each of a plurality of rows in the effective pixel array 111 successively. And, all pixels in selected row are reset at a time or read out into vertical signal lines at a time. Both of the resetting operation and the operation for reading out into vertical signal lines are under control of a vertical scanning circuit 113.
Output signals from the selected row of pixels are stored in a readout circuit 115, until they are delivered sequentially. This operation for delivering the signals from the readout circuit 115 sequentially is controlled by a horizontal scanning circuit 114.
The output signal from the readout circuit 115 has an offset component corresponding to a black level, which represents a state in which there is no incident light. The amount of this offset component differs among different circuit configurations of pixels and readout circuit.
Therefore, the offset component corresponding to a black level is subtracted from each signal of pixels in the effective pixel array 111 in the manner described below. Referring again to FIG. 1, a light-shielded pixel array 112 covered with a light-shielding film to prevent incidence of light is arranged around the effective pixel array 111. Shielded pixels are read out in the same way as effective pixels. Output signals from the shielded pixels are passed through the readout circuit 115, then separated from the effective pixel signals, and then stored in an analog storage circuit 116. In a signal level adjusting circuit 117, an offset component stored in the analog storage circuit 116 is subtracted from the effective pixel signal.
Then, the output signal from this signal level adjusting circuit is applied to an A/D converter (not shown). At this time, an additional offset voltage may be added such that the black level of the output signal from the signal level adjusting circuit matches the level of the lower-voltage side of the input voltage range of the A/D converter. Also, in this case, the black-level signal held in the analog storage circuit 116 is utilized.
In some sensors, a black-level signal generation circuit may be disposed apart from the pixel array. FIG. 2 is a block diagram schematically showing such image sensor accompanied with a black-level signal generation circuit, which is disclosed in Japanese Patent Publication Hei 5-23549. In this image sensor, a second photo-sensor cell 204 coated with a light-shielding film is provided besides photo-sensor cells constructing a photo-sensor array 201. A black-level signal is read out from the photo-sensor cell 204, being synchronized to the read-out timing signal of the photo-sensor array 201, and stored in a storage circuit 205. Then, a light information signal is obtained by subtracting the black-level signal stored in the storage circuit 205 from the output signal read out sequentially from the photo-sensor array 201 using a differential amplifier 202.
In the conventional MOS-based image sensors according to the prior arts described above, it is necessary that the black-level signal and the effective pixel signal should be separated from each other and that the black-level signal should be stored in the analog storage circuit. Therefore, a timing signal generating circuit for producing a timing pulse for separating the black-level signal from the effective pixel signal and the analog storage circuit are necessitated.
FIG. 3 is a block diagram schematically showing an image sensor creating a black-level signal by an electric circuit, which is disclosed in the above Japanese patent publication. A light information signal is obtained by subtracting a black-level signal formed using a constant current source 203 constructed from a constant voltage source and a variable resistor from an output signal read out from a photo-sensor array 201 using a differential amplifier 202. In the method, however, after the chip of the image sensor is completed or after the chip is incorporated into an imager, it is necessary to adjust the voltage value of the constant voltage source and/or the resistance value of the variable resistor for each individual commercial product of the chip to obtain a suitable level for the black-level signal.
Therefore, it is a main object of the present invention to provide a solid state image sensor having an improved black-level signal generation circuit.
It is another object of the present invention is to provide a CMOS image sensor that has a simplified black-level signal generation circuit.
It is still another object of the present invention is to provide a MOS-based image sensor that does not need an analog storage circuit that would be normally used to store a black-level signal.
It is another object of the present invention to provide a MOS-based image sensor that does not need a timing signal generating circuit that would be usually required to separate a black-level signal from an effective pixel signal.
It is still another object of the present invention to provide a MOS-based image sensor having a black-level signal forming means that makes it unnecessary to adjust a voltage value of a constant voltage source and/or a resistance value of a variable resistor for each individual image sensor chip singly.
An image sensor according to the present invention includes a plurality of pixels and a black-level signal generation circuit. Each pixel has a photo-detector or photo-sensor that generates charge in response to a light illuminated thereto and a signal producing circuit that produces an electrical signal in response to the charge generated by the photo-sensor. In accordance with the present invention, the black-level signal generation circuit is constructed to produce a black-level signal that is substantially equivalent to such a signal that is produced by the pixel upon being subjected to a reset state.
In a pure technical aspect, the black-level signal is required to have the so-called xe2x80x9cdark currentxe2x80x9d component that is generated by the photo-sensor itself and a signal component that is inherently produced by the signal producing circuit irrespective of the charge generated by the photo-sensor. Such signal component will be called xe2x80x9cinherent signal componentxe2x80x9d hereinafter. The dark current component corresponds to such a signal that is generated by the photo-sensor that is activated under a light-shielded condition, i.e., under a dark condition. These two components are irrelevant to the pure signal that is truly responsive to the charge which is generated by the photo-sensor according to the intensity of the light.
The inventors of the present invention have, however, recognized that the dark current component is not always necessary for some applications of an image sensor. For example, in mobile gears such as handy PCs or cellular phones equipped with an image sensor to transmit images such as pictures to a receiver, the picture quality of such images is not required to be high. Rather, a high cost-performance is required to such an image sensor that is to be installed into the mobile gears. In addition, the dark current component itself has been lowered in accordance with the progress in device structure and/or manufacturing process of a photo-sensor. Based on this recognition, the dark current component is no longer taken into account and thus can be ignored. As a result, the black-level signal generation circuit according to the present invention is so arranged as to generate, as a black-level signal, such a signal that is substantially equivalent to a signal which is produced by the pixel upon being subjected to a reset state. This signal thus corresponds to the xe2x80x9cinherent signal componentxe2x80x9d.
Since the dark current component is not used as a black-level signal, any photo-sensor that is to be shielded from the light for obtaining the dark current component is no longer necessary. Even a photo-sensor used for generating a black-level signal may be omitted. Moreover, any timing control or any analog storage circuit, which would be otherwise required in the prior art sensors, is not required. The black-level signal generation circuit according to the present invention can produce a black-level signal as a DC signal. Thus, the image sensor according to the present invention presents a high cost performance, which is one of key factors for mobile gears.
According to the another aspect of the present invention, there is provided a black-level signal generation circuit for a MOS-based image sensor, wherein the black-level signal generation circuit is equivalent in circuit configuration to any one of pixels producing effective pixel signals and any one of readout portions for reading out the effective pixel signals. This black-level signal generation circuit can constantly produce an output signal equivalent to an effective pixel signal delivered from any one of readout portions when the pixels are in a reset state. The black-level signal generation circuit may contain all circuit elements which are equivalent in function to circuit elements included in any one of the pixels and any one of the readout portions delivering effective pixel signals. However, if some of these circuit elements can be omitted or replaced with electric conductors without substantially affecting the value of the level of the produced black-level signal, then such circuit elements can be omitted or replaced with the electric conductors. A power-supply voltage or a ground potential is supplied to the gates of MOS transistors included in the black-level signal generation circuit whose corresponding MOS transistors are turned xe2x80x98ON/OFFxe2x80x99 in any one of the pixels and any one of the readout portions, whereby the MOS transistors are kept xe2x80x98ONxe2x80x99. If there is any light-receiving element within the black-level generation circuit, the light-receiving element is maintained in a reset state. In the present invention, more than one black-level signal generation circuit can be formed. Only one black-level signal generation circuit can be also used.
The foregoing and other objects of the present invention, together with its novel features, will become more apparent when the following detailed description is read with reference to the accompanying drawings. Note that the drawings are for illustrative purposes only and not intended to limit the scope of the invention.